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r_ 1' I CrMounding Analysis Reference is made to the encountered groundwater table at approximately nine feet below existing.grade. lasThis depth to the water table can be considered comparatively shallow and may induce aphenomenon known the"mounding effect" in which percolating effluent encounters the water table and cannot disperse laterally in a certain:time frame. Consequently, around forms under the disposal field creating saturated flow conditions and decreasing the distance the effluent must travel under unsaturated flow for effluent treatment to occur. There must be sufficient distance between the soil/effluent interface and the highest anticipated depth to groundwater for sufficient treatment to occur, which is generally aceepted to be five feet. If the soil-effluent interface is two-to-three feet below grade,then the water table must be at least seven-to- -eight feet deep. An equation developed by Finnemore and Hantzsche(1983) is used below to predict the long-term maximum rise of the mound: h =H+Z,,,+2 where h =distance from boundary to mid-point of the long-term mound,,in ft H=-height'of stable groundwater table above impermeable boundary, in'ft -Z;n= long-term maximum rise of the mound, in ft . Substitutingknown and estimated values for the variables,. a abler, we find the following: H =The height of stable groundwater above an impermeable'boundary is estimated to be 100 ft. Therefore, it will be assumed long-term rise of the.mound is four ft: h = 100+2= 102 ft. Z1-0— C 5.n - m _ l l_ - l A I 141 I Kb '1 .Sy) where: Q=average daily flow in W/day A=area of disposal field in ft' C=mounding equation constant L length of disposal field in ft K=horizontal permeability of soil.in ft/day n=mounding equation exponent Sy=specific yield of receiving soil in percent t. =time since the beginning of wastewater application in days Substituting known constants for;the variables,we find the following: Q:=2,493 gpd(From Max.flow volume talcs.,Page 15)-7.48 gals/ft'=333 ft'/day A 9,600 W(From disposal area sizing talcs,Page 19) _ C=Length to width ratio 1.7,therefore, C=2.0748 L=128ft K=Using average vertical:permeability as most conservative=36 min/in: 1440 min/day 36min/in=3.3 ft/day h= 102 (See above) n=Length to width ratio �2,therefore,n= 117552 Sy=0.28 From Soil Texture Analyses t =:3,650 days(10 yrs) Z,n =0.2597 x 438.37 x 0.00605 x 3.19=2.19 ft /t appears that-the.maximum mound height that may occur under the chambered filter bed is 2.2 feet. This .would.leave a distance of approximately five.feat between the soil/effluent interface and the top of the theoretical mound: soil/effluent interface =2.ff below existing grade+5 ft'se'paration distance= 7 ft below :grade. The mound may rise to a depth of: 2.2 ft rise of mound 7 ft below grade+2.2=9.2 ft-the 'approximate depth.to the water table. l0 , . Chesney Consulting